Override version of Maven Site Plugin for JDK 9+181 compatibility

See gh-16400

Author: wilkinsona

spring-boot-project/spring-boot-tools/spring-boot-maven-plugin/pom.xml

@@ -189,6 +189,7 @@
 					<plugin>
 						<groupId>org.apache.maven.plugins</groupId>
 						<artifactId>maven-site-plugin</artifactId>
+						<version>3.7.1</version>
 						<executions>
 							<execution>
 								<id>generate-site</id>